Tính (theo mẫu):

Mẫu: $\frac{3}{4}\times 5=\frac{3}{4}\times \frac{5}{1}=\frac{3\times 5}{4\times 1}=\frac{15}{4}$

Ta có thể viết gọn như sau: $\frac{3}{4}\times 5=\frac{3\times 5}{4}=\frac{15}{4}$

Mẫu: $4\times \frac{2}{9}=\frac{4}{1}\times \frac{2}{9}=\frac{4\times 2}{1\times 9}=\frac{8}{9}$

Ta có thể viết gọn như sau: $4\times \frac{2}{9}=\frac{4\times 2}{9}=\frac{8}{9}$

$$\begin{gathered} a)\hspace{0.17em}\frac{5}{6}\times 7=\mathrm{.............................................} \\ b)\hspace{0.17em}\frac{7}{10}\times 3=\mathrm{.............................................} \\ c)\hspace{0.17em}5\times \frac{4}{21}=\mathrm{.............................................} \\ d)\hspace{0.17em}2\times \frac{5}{9}=\mathrm{.............................................} \end{gathered}$$